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Date: Tue, 16 Jul 2013 14:43:15 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hughd@google.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Colin Cross <ccross@android.com>, David Rientjes <rientjes@google.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] mm: shift VM_GROWS* check from mmap_region() to
 do_mmap_pgoff()
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In-Reply-To: <20130714165451.GA20847@redhat.com>
References: <20130714165432.GA20828@redhat.com>
	<20130714165451.GA20847@redhat.com>
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On Sun, 14 Jul 2013 18:54:51 +0200 Oleg Nesterov <oleg@redhat.com> wrote:

> mmap() doesn't allow the non-anonymous mappings with VM_GROWS* bit set.
> In particular this means that mmap_region()->vma_merge(file, vm_flags)
> must always fail if vm_flags & VM_GROWS. So it does not make sense to
> check VM_GROWS* after we already allocated the new vma, the only caller,
> do_mmap_pgoff(), which can pass this flag can do the check itself.
> 
> And this looks a bit more correct, mmap_region() already unmapped the
> old mapping at this stage. But if mmap() is going to fail, it should
> avoid do_munmap() if possible.
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1327,6 +1327,9 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>  		}
>  	}
>  
> +	/* Only MAP_PRIVATE|MAP_ANONYMOUS can use MAP_GROWS */
> +	if ((vm_flags & VM_MAYSHARE) && (vm_flags & (VM_GROWSDOWN|VM_GROWSUP)))

That is of course vm_flags&(VM_MAYSHARE|VM_GROWSDOWN|VM_GROWSUP), but
that perhaps is less clear.

> +		return -EINVAL;

I had to stare for a while but yes, the change looks OK to me.